Summary

During the last 30 years, production of the main food crops has
doubled. This increase of production has mainly been achieved by
introduction of high-yielding varieties, irrigation, and the use of
fertilizers and pesticides. Due to this increase, the share of people in
developing countries with insufficient average food supply has
decreased from 74 % in 1962 to 6 % in 1988, representing 230 million
people. In many regions of the world, the intensification of crop
production has led to deterioration of soil fertility, erosion, salinization,
reduction of biodiversity, and other deleterious side-effects. The use of
pesticides has more than tripled since 1970 and is a growing concern
especially in developed countries. Despite the intensive use of chemical
crop protection methods, the losses due to pests, pathogens and
weeds are more than 40 % of attainable production, representing a
value of more than 240 billion US$.

Genetic engineering offers new possibilities for the breeding of plant
varieties with increased resistance to pests and pathogens. New
resistant varieties may lessen the dependence on pesticides and help
securing sufficient crop yields in the future.

In plant genetic engineering, genes from different organisms (other
plants, bacteria, viruses, etc.) are transferred into the genome of a
plant cell1. The bacterium Agrobacterium tumefaciens is
frequently used as a vehicle for the introduction of foreign DNA into the
plant genome. In nature, these bacteria transfer some of their genes into
the plant genome, thereby inducing a plant disease which leads to
production of compounds used by the bacteria. In genetic engineering,
the genes causing disease are replaced by genes conferring other traits.
For some plants, e.g. wheat or maize, Agrobacterium-mediated gene
transfer is difficult or not possible. In these cases, a technique called
particle bombardment is often applied. In this method, gold or tungsten
particles of about 5 µm in diameter are coated with DNA and shot into
plant cells, where the DNA is released and incorporated into the plant
genome. After incorporation of the foreign gene, a plant is regenerated
from an engineered cell, and the traits coded for by the transferred
gene are expressed by the plant.

Resistance of transgenic plants to insect pests or diseases has been
achieved in more than 20 different crops, including maize, potato,
squash, cotton, soybean, oilseed rape, tomato, tobacco, alfalfa, rice,
barley and others. Very high levels of resistance to insect pests and
viral diseases have been reached, while examples of successful
protection to bacterial and fungal diseases are still scarce.

Insect resistance has mostly been obtained by using a gene derived
from the common soil bacterium Bacillus thuringiensis. This bacterium
produces a protein called Bt toxin which is toxic for certain insects.
Intensive investigations have led to a detailed knowledge of the
mechanism and specificity of toxin activity. In several studies, no effect
of Bt toxin on humans, other mammals, and most non-target insects
could be shown. Transgenic plants expressing Bt toxin were found to be
protected against repeated heavy infestations of the target insect pest
which totally devastated non-transgenic control plants. Other approaches
to insect resistance focus on the use of genes which are part of the
natural defense system of plants. The products of these genes interfere
with insect digestion. For example, plant-derived protease inhibitors
prevent protein degradation, and amylase inhibitors block starch-
degrading enzymes in the insect midgut. Some of these strategies
have proven to be effective and may soon be used in the development
of commercial varieties.

Virus resistance is mostly achieved by introducing gene sequences
derived from pathogenic viruses into the crop genome. The introduction
of genes coding for viral coat proteins has been very successful. During
the last years, this strategy has led to a number of crop varieties
resistant to important plant viruses. More recently, also other viral
genes were found to confer resistance, e.g. replicase genes, defective
viral genes or antisense coat protein genes. The mechanisms of
resistance are not yet completely understood.

Strategies applied to achieve fungal resistance make use of plant
genes acting on different levels of the plant defense system against
pathogens. Several of these strategies have led to increased resistance,
but so far the level of protection was mostly to low to be of agronomic
importance. Chitinase and glucanase genes coding for enzymes which
break down fungal cell walls have been used in several crops including
rice and have led to significant protection in some cases. The growing
understanding of plant defense mechanisms is expected to lead to
increased levels of protection in the near future.

Also methods investigated to obtain resistance to bacteria have not
led to high levels of protection yet. Reduction of disease development
in tobacco was achieved by transferring a cecropin gene derived from
the Giant silk moth. Cecropins are produced by insects to fight pathogen
attack and had a similar effect in some plants. Other partially successful
strategies make use of genes which code for toxindetoxifying enzymes
or plant genes involved in the response to pathogen attack.

Besides genetically engineered plants, also viruses and bacteria have
been genetically altered in order to develop new crop protection methods.
Baculoviruses are insect pathogens which have been used as a biological
pesticide since the 1930s. As these viruses may take weeks to kill their
host after infection, their usefulness has been limited. By transferring
genes coding for insect-specific toxins, insect hormones and insect
enzymes into the virus genome, the killing time has been reduced by
up to 50 %, which is not enough to achieve sufficient protection. Bacillus
thuringiensis (Bt) toxin genes have been introduced into different
bacteria for Bt toxin delivery to insect pests. In one approach,
transgenic bacteria expressing Bt toxin are killed and then sprayed on
the crop plants like a pesticide. Another approach uses bacteria living
inside of plants for Bt toxin delivery.

The safety aspects of transgenic organisms have been discussed
and investigated since the first successful gene transfer in the early 1970s.
The release of transgenic plants is subject to different legal regulations.
Before a transgenic crop may be released, potential hazards like the
possibility of gene transfer to other plants or microorganisms, weediness
of the engineered crop, and the expression of undesirable traits resulting
from secondary effects of the gene insertion are examined. Also possible
toxic and allergenic effects are analyzed, especially if the engineered
plant is destined to serve as a food crop. So far, no deleterious effects
of transgenic plants or other organisms have been reported.

Between 1986 and 1993, more than 1000 field releases of transgenic
plants were conducted in 32 countries, and the number is increasing
rapidly. The USA and Canada account for more than half of all trials
recorded. Thirty-eight different plant species have been tested. Potato,
oilseed rape, tobacco, maize, tomato and sugar beet constitute together
70 % of all trials. Among these crops, about one third of all trials
evaluated pest and disease resistance (16 % virus resistance, 13 %
insect resistance, 3 % fungal resistance and 1 % bacterial resistance).
The trait most commonly tested was herbicide tolerance (34 %). Other
traits examined were quality improvement (20 %) and marker genes
(10 %).

In the USA, permits for commercialization of 10 transgenic crop
varieties have been granted as of July 1995: Insect resistant potato,
maize and cotton expressing Bt toxins, virus resistant squash expressing
viral coat proteins, two tomato varieties with extended shelf-life, one
tomato variety with enhanced process value, oilseed rape with altered
oil composition, and herbicide tolerant cotton and soybean. In Canada,
herbicide tolerant flax and oilseed rape have been commercialized. In
the European Union, a herbicide tolerant tobacco variety has obtained
a permit for market introduction.

Several other transgenic crops are approaching commercialization.
In the field of pest and disease resistance, it is likely that more insect
resistant crops expressing Bt toxins or virus resistant crops engineered
with viral genes will enter the market in the near future. Within some
years, varieties with enhanced resistance against fungal and bacterial
pathogens may also become available. Other applications of transgenic
plants which may reach the marketplace within some years include e.g.
cotton with altered fibers, crops with improved nutritional value and
plants producing biodegradable plastic, cheap vaccines and
pharmaceuticals.